Separator with long-term stability for an electrochemical cell

ABSTRACT

A separator for an electrochemical cell, comprising (A) a flexible perforate support, (B) a porous first ceramic material which fills the perforations in the support and which (i) has a pore structure which is characterized by an average pore size, and (ii) is suitable for receiving an ion-conducting electrolyte, wherein (C) the electrolyte-contactable pore surface of the first porous ceramic material is covered with fine particles of a further material to extend the use life, the average size of the fine particles being in the range from 0.5 to 30% and preferably in the range from 1 to 15% of the average pore size of the ceramic material.

The present invention relates to a separator for an electrochemicalcell, to a process for producing such a separator and also to anelectrochemical cell which comprises such a separator.

As used herein, electrochemical cell or battery refers to batteries andaccumulators (secondary batteries) of any kind, especially alkali metalsuch as for example lithium, lithium ion, lithium polymer, and alkalineearth metal batteries and accumulators, in the form of high energy orhigh power systems.

Electrochemical cells comprise electrodes of opposite polarity which areseparated from each other by a separator while maintaining ionconductivity.

A separator is conventionally a thin porous electro-insulating materialpossessing high ion permeability, good mechanical strength and long-termstability to the chemicals and solvents used in the system, for examplein the electrolyte of the electrochemical cell. In electrochemicalcells, the separator should fully electronically insulate the cathodefrom the anode. Moreover, the separator has to be permanently elasticand to follow movements in the system, for example in the electrode packin the course of charging and discharging.

The separator is a crucial determinant of the use life of the system inwhich it is used, for example the use life of an electrochemical cell.The development of rechargeable electrochemical cells or batteries istherefore influenced by the development of suitable separator materials.General information about electrical separators and batteries may befound for example at J. O. Besenhard in “Handbook of Battery Materials”(VCH-verlag, Weinheim 1999). High energy batteries are employed invarious applications where the decisive requirement is that a very largeamount of electrical energy be available. This is the case for examplewith traction batteries, but also with auxiliary power systems. Theenergy density is frequently reported in this field per unit weight[Wh/kg] or per unit volume [Wh/L]. Currently, high energy batteriesreach energy densities of 350 to 400 Wh/L and of 150 to 200 Wh/kg. Thepower levels which such batteries are expected to supply are not thathigh, so that compromises can be made with regard to the internalresistance. In other words, the conductivity of the electrolyte-filledseparator for example does not have to be as large as in the case ofhigh power batteries, for example, so that the way is open to the use ofother separator designs.

High energy systems, for instance, can even utilize polymer electrolyteswhose conductivity at 0.1 to 2 mS/cm is fairly low. Such polymerelectrolyte cells cannot be used as high power batteries.

Separator materials for use in high power battery systems have to havethe following properties:

They need to be very thin to ensure low specific space requirements andto minimize the internal resistance. To ensure these low internalresistances, it is important that the separator also has a highporosity. Further, they have to be light in order that a low specificweight may be achieved. In addition, wettability has to be high, sinceotherwise unwetted dead zones are formed.

There are many applications, especially mobile applications, where verylarge amounts of energy are required, eg in traction batteries. Thebatteries in these applications thus store large amounts of energy inthe fully charged state. The separator has to be safe for thesebatteries, since very large specific electric energy quantities aretransported in these batteries. These energies must not in any way bereleased in an uncontrolled fashion in the event of a dysfunction of thebattery or of an accident, since this would ineluctably lead to the cellexploding and going up in flames.

Currently used separators consist predominantly of porous organicpolymeric films or of inorganic nonwoven web materials, for example webscomposed of glass or ceramic materials or else ceramic papers. These aremanufactured by various companies. Important producers here are:Celgard, Tonen, Ube, Asahi, Binzer, Mitsubishi, Daramic and others.

Separators composed of inorganic nonwovens or of ceramic paper aremechanically very unstable and tend to short circuit, with the resultthat it is impossible to achieve a long use life.

A typical organic separator consists for example of polypropylene or ofa polypropylene-polyethylene-polypropylene composite. A substantialdisadvantage of these organic polyolefin separators is their low thermalstability limit of below 150° C. Even brief attainment of the meltingpoint of these polymers leads to substantial melting of the separatorand to short circuiting in the electrochemical cell utilizing such asseparator. The use of such separators is therefore generally not safe.This is because when higher temperatures are attained, especiallytemperatures of above 150° C. or even 180° C., these separators aredestroyed.

As well as this instability at high temperatures, polymer-basedseparators have further serious disadvantages with regard to chemicalstability. The polymers in the electrochemical cells are slowly butcontinually attacked by contact with the electrodes even at normaloperating and charging temperatures such as room temperature. Problemsarise in particular with the use of such separators in electrochemicalcells which utilize lithium. The polymer is slowly attacked at thecontact surface of the separator with the lithium or the lithiatedgraphite. Moreover, polymeric separators are also attacked in theinterior of the separator by the materials which form during theoperation of an electrical cell. As a result, these separators can nolonger reliably protect the electrodes against short circuiting. The uselife is reduced as a result. In addition, the capacity of anelectrochemical cell which utilizes such separators decreases over time.

There have been initial attempts to use inorganic composite materials asseparators in order to overcome these disadvantages. For instance, DE198 38 800 C1 proposes an electrical separator having a compositestructure that comprises a sheetlike flexible substrate having amultiplicity of openings and having a coating on the substrate. Thematerial for the substrate is selected from metals, alloys, plastics,glass and carbon fiber or the combination thereof, and the coating is atwo-dimensionally continuous porous electrically nonconducting ceramiccoating. The use of a ceramic coating promises thermal and chemicalstability. The separators, which as exemplified are supported by acarrier or substrate composed of electrically conductive material,however, have been determined to be unsuitable for electrochemicalcells, since the coating has proved impossible to produce over a largearea without flaws at the thickness described and consequently shortcircuiting can occur very easily. Nor are such thin metal fabrics asrequired for very thin separators commercially available. We were ableto show in previous work (DE 101 42 622) that a material comprising asheetlike flexible substrate having a multiplicity of openings andhaving a coating on and in this substrate, the material of the substratebeing selected from woven or non-woven nonelectroconductive fibers ofglass or ceramic or a combination thereof and a coating being a porouselectrically insulating ceramic coating, can be used to produce aseparator which has a thickness of less than 100 μm and is bendable, theresulting separator having a sufficiently low resistance in connectionwith the electrolyte and yet possessing sufficiently good long-termstability. The separator described in DE 101 42 622 possesses very highconductivity, but the separator described therein still does not meetthe requirements of an industrially useful separator with regard tothickness and weight and also safety.

In DE 102 08 277, the weight and thickness of the separator was reducedby using a polymeric nonwoven, but the separator embodiments describedtherein likewise still do not meet all requirements of the separator fora lithium high energy battery, especially because particular emphasiswas placed in this application on very large pores for the separator.But the particles described therein, up to 5 μm in size, do not permitthe production of separators which are 10-20 μm in thickness, since onlyfew particles would come to rest on top of each other here. As a result,the separator would inevitably have a large defective and disruptivesite density (eg holes, cracks, . . . ). Moreover, the large particlesin this reference consist of Al₂O₃ and ZrO₂. Owing to the high densityof these ceramics, these separators have high basis weight, whichreduces the mass-based specific energy density in Wh/g.

But even this more or less inorganic separator will react over time withthe electrodes or with other materials present in the battery. Thislimits the use or service life of the battery, especially at elevatedtemperatures, in particular in the course of the operation and in thecourse of the storage of batteries which are equipped with suchseparators.

It therefore is an object of the present invention to provide aseparator for an electrochemical cell that has an increased use life.

This object is achieved by a separator for an electrochemical cell,comprising

-   (A) a flexible perforate support,-   (B) a porous first ceramic material which fills the perforations in    the support and which    -   (i) has a pore structure which is characterized by an average        pore size, and    -   (ii) is suitable for receiving an ion-conducting electrolyte,        characterized in that-   (C) the electrolyte-contactable pore surface of the first porous    ceramic material is covered with fine particles of a further    material to extend the use life, the average size of the fine    particles being in the range from 0.5 to 30% and preferably in the    range from 1 to 15% of the average pore size of the ceramic    material.

Preferably, the fine particles consist of a ceramic material.

Advantageously, such a separator where the pores of a ceramic compositeare coated with fine particles has a longer use life than a ceramiccomposite which is not coated with fine particles. Surprisingly, such aseparator according to the present invention also exhibits high ionconductivity, even though the volume available for the electrolyte isreduced in size by the fine particles.

The material for the fine particles can be identical to or differentfrom the porous ceramic material. In a preferred embodiment of theseparator of the present invention, the material of the fine particlesis different from the porous ceramic material. The fine particles maycomprise for example SiO₂, Al₂O₃, ZrO₂ or sic.

In a particularly preferred embodiment of the present invention, thefine particles comprise Li₂CO₃, Li₃N, LiAlO₂ or Li_(x)Al_(y)Ti_(z)(PO₄)₃where 1≦x≦2, 0≦y≦1 and 1≦z≦2. These particles advantageously enhance theconductivity of the separator for ions.

The separator of the present invention may comprise an electrolyte forion conductance. Preference is given to an alkali and alkaline earthmetal ion conductance and particular preference is given to a lithiumion conductance.

The separator of the present invention may have the fine particlesincorporated into the porous first ceramic material and exposed on thepore surface. In a further embodiment of the invention, the porous firstceramic material is merely coated with the fine particles on the poresurface.

The ceramic material of the separator according to the present inventionpreferably has an average pore size in the range from 50 nm to 5 μm. Theporous ceramic material comprising fine particles may further have aporosity in the range from 10% to 70%. Preference is given to a porosityin the range from 20% to 50%. The ceramic material of the separatoraccording to the present invention preferably comprises an oxide ofzirconium, silicon and/or preferably aluminum.

In a preferred embodiment of the invention, the first ceramic materialof the separator is producible by solidifying a slip which containsparticles having a large average particle size which determine the porestructure of the ceramic material and also particles having a smalleraverage primary particle size which adhere the large particles togetherin the course of the solidification of the slip.

The separator of the present invention preferably comprises a perforatesupport which comprises polymeric fibers, glass or ceramic.

In a preferred embodiment of the present invention, the perforatesupport comprises fibers, preferably selected from fibers of polyamide,polyacrylonitrile, polyester, eg polyethylene terephthalate (PET) and/orpolyolefin, eg polyethylene (PE) or polypropylene (PP), glass fibers orceramic fibers. When the perforate support comprises polymeric fibers,polymeric fibers other than those mentioned above may be used, providedthey not only have the thermal stability required for producing theseparators but also are stable under the operating conditions in anelectrochemical cell, preferably a lithium battery. In a preferredembodiment, the separator according to the invention comprises polymericfibers having a softening temperature of above 100° C. and a meltingtemperature of above 110° C.

The support may comprise fibers and/or filaments from 1 to 150 μm andpreferably from 1 to 20 μm in diameter and/or yarn from 3 to 150 μm andpreferably from 10 to 70 μm in diameter.

In a further embodiment of the invention, the support is a nonwovenhaving a pore size from 5 to 500 μm and preferably from 10 to 200 μm.

The separator of the invention may be from 10 to 1 000 μm, preferablyfrom 10 to 100 μm and most preferably from 10 to 50 μm in thickness.

The separator of the invention is by virtue of its composite structureable to tolerate a bending radius down to 100 mm, preferably down to 20mm and most preferably down to 1 mm. The separator of the presentinvention is by virtue of its construction according to the inventionvery useful for electrochemical cells possessing high capacity and highenergy density. More particularly, the separator according to theinvention is useful for electro-chemical cells which are based on thetransference of alkali and/or alkaline earth metal ions, such aslithium-metal and lithium ion batteries for example. It is thereforeadvantageous when the separators also possess the protective measuresspecific to this application, such as shutdown and meltdown with a highshort circuiting temperature. Shutdown refers to a measure in which theseparator may have incorporated into it materials which are to beselected for certain operating temperatures and melt easily, such asthermoplastic materials for example. In the event of a rise in theoperating temperature due to upsets such as overcharging or external orinternal short circuiting, such easy-melting materials can melt and plugthe pores of the separator. Thus the ion flux through the separator ispartially or completely blocked and a further rise in the temperature isprevented. Meltdown refers to the property that the separator will meltcompletely at a short circuiting temperature. Large areas of theelectrodes in an electrochemical cell can then come into contact andshort circuit. A very high short circuiting temperature is desirable forsafe operation of an electrochemical cell possessing high capacity andenergy density. The separator according to the invention has animportant advantage in this respect. This is because the ceramicmaterial which adheres to the perforate support in the case of theseparator of the present invention has a melting point which is farabove the safety-relevant temperature range for electrochemical cells.The separator of the present invention therefore possesses outstandingsafety. This is because, in a preferred safe embodiment, it is stableunder service conditions of not less than 50° C. More preferably, it isstable at not less than 100° C., 150° C. and most preferably at not lessthan 180° C.

Polymeric separators provide for example the safety demanded at presentfor lithium batteries by stopping any ion transport through theelectrolyte from a certain temperature (the shutdown temperature ofabout 120° C.). This happens because at this temperature the porestructure of the separator collapses and all the pores are closed.Because ions can no longer be transported, the dangerous reaction whichcan lead to explosion ceases. But if the cell continues to undergoheating because of external circumstances, then the breakdowntemperature is exceeded at about 150 to 180° C. At this temperature, theseparator melts and contracts. Direct contact then comes about betweenthe two electrodes at many places in the battery cell, leading tointernal short circuiting over a large area. This leads to anuncontrolled reaction which ends with an explosion of the cell, or theresultant pressure is released by an overpressure valve (a burstingdisk), frequently with signs of fire.

In a particularly preferred embodiment of the invention, the flexibleperforate support of the separator comprises polymeric fibers. Thishybridic separator, comprising a combination of inorganic components andof polymeric support material, undergoes shutdown when the hightemperature causes the polymer structure of the support material to meltand to penetrate into the pores of the inorganic material, therebyclosing them. But meltdown does not occur with the separator accordingto the invention. By virtue of its shutdown mechanism in the batterycells, the separator according to the invention thus meets therequirements, voiced by various battery manufacturers, for a safetyswitchoff mechanism. The inorganic particles ensure there can never be ameltdown. It is thus ensured that there are no operating states wherelarge-area short circuiting can occur.

It may be preferable for the separator to comprise an additional,noninherent shutdown mechanism. This additional, noninherent shutdownmechanism may be achieved for example when a very thin layer of waxy orpolymeric shutdown particles, which melt at a desired shutdowntemperature, is present on or in the separator. Particularly preferredmaterials for shutdown particles include for example natural orartificial waxes, low-melting polymers, for example polyolefins, thematerial for the shutdown particles being chosen so that the particlesmelt at the desired shutdown temperature and close the pores of theseparator to prevent further ion flux.

Preferably, the shutdown particles have an average particle size (D_(w))which is not less than the average pore size (d_(s)) of the pores of theporous inorganic layer of the separator. This is advantageous inparticular because this prevents penetration and closing of the pores ofthe separator layer that will result in a reduction in the pore volumeand hence in separator performance and also battery performance. Thethickness of the shutdown particle layer is only critical insofar as anexcessively thick layer would increase the resistance in the batterysystem unnecessarily. To achieve safe shutdown, the shutdown particlelayer should have a thickness (z_(w)) which is approximately in therange from the average particle size of the shutdown particle (D_(w)) upto 10 D_(w) and preferably in the range from 2 D_(w) to D_(w). A thusequipped separator comprises a primary safety feature. In contrast tothe all-organic separator materials, however, this separator cannot meltcompletely and there can never be a meltdown. These safety features arevery important for high energy batteries owing to the very large energyquantities and therefore are frequently mandated.

The separator according to the invention is also very safe in the eventof internal short circuiting due to an accident for example. If, forexample, a nail would puncture a battery, the following would happen,depending on the type of separator: a polymeric separator would melt atthe site of puncture (a short circuit current flows through the nail andcauses it to heat up) and contract. As a result, the short circuitinglocation will become larger and larger and the reaction would get out ofcontrol. Only the polymeric substrate material at most would melt at thehybridic separator of the present invention, but not the inorganicseparator material. So the reaction in the interior of the battery cellfollowing such an accident would proceed much more moderately. Thisbattery is thus distinctly safer than one with a polymeric separator.This is an important factor in mobile applications in particular.

The above-described inventive separator for an electrochemical cell maybe produced by the following processes. The first of these processescomprises the following steps:

-   (a) applying a dispersion as a thin layer onto and into a woven    and/or nonwoven, the dispersion comprising    -   (a1) large ceramic particles whose average particle size        provides a pore structure to the layer that is characterized by        an average pore diameter,    -   (a2) fine particles whose average particle size is in the range        from 0.5 to 30% and preferably in the range from 1 to 15%, of        the average particle size of the ceramic material, and also    -   (a3) optionlly, ceramic particles having an average primary        particle size which is substantially less than the average        particle size of the ceramic particles as per (a1) and (a2), and-   (b) solidifying the dispersion at a temperature from 100° C. to    680° C. to form a separator.

This process has the advantage that it can be carried out in few steps.

The second process for producing an electrochemical cell separatoraccording to the present invention comprises the following steps:

-   (i) providing a composite formed from a perforated support,    preferably a woven and/or nonwoven, and also a porous ceramic    material whose pore structure is characterized by an average pore    size,-   (ii) treating the composite with a dispersion of fine particles    having an average particle size in the range from 0.5 to 30% and    preferably in the range from 1 to 15% of the average pore size in a    dispersion medium so that the electrolyte-accessible pore surface of    the composite is coated with the dispersion and the dispersion    preferably contains from 1 to 25% by weight, especially from 5 to    15% by weight of fine particles;-   (iii) drying the dispersion at a temperature in the range from    100° C. to 680° C. so that the coated pore surface is coated with    the fine particles.

This process has the advantage that only the pore surface becomescovered with the fine particles. Furthermore, the dispersion of fineparticles in step (ii) is independent of the chemistry of a slip forproducing the composite.

In the second process, the composite can be a separator which isobtainable by the first process.

In either of the processes described, the dispersion may contain one ormore additional components which are selected from adhesion promoters,dispersing assistants, agents for setting the viscosity, agents forsetting the flow properties or other customary assistants for producingdispersions. The adhesion promoters, for example the functionalizedsilanes described hereinbelow, are particularly advantageous in thatthey are able to bind the first ceramic material particularly firmly tothe flexible perforate support composed of polymer, glass or ceramic.For example, functionalized silane adhesion promoters may beparticularly preferable when the support comprises a polymeric material.But adhesion promoters may similarly advantageously be sols whichpreferentially bind the fine particles firmly to the first porousceramic material. Advantageously, the dispersion in the processes forproducing the first porous ceramic material comprises an adhesionpromoter selected from the group consisting of aluminum oxide, siliconoxide and zirconium oxide. These oxides may be present in hydrolyzedform or nonhydrolyzed form of appropriate precursor compounds in adispersion. The dispersion medium for the dispersions of the processesaccording to the present invention may contain water or water-containingsolvents, for example alcohols, esters, ketones, etc, and the fineparticles may be hydrolysis-stable element oxide particles.

But the dispersion medium may also be an anhydrous organic solvent, forexample a hydrocarbon, N-methyl-pyrrolidone (NMP), dimethyl sulfoxide(DMSO), etc, especially when the fine particles comprisehydrolysis-sensitive materials, such as Li₃N for example.

The fine particles should be present in thoroughly dispersed form. Thiscan be accomplished by prolonged stirring, the use of ultrasonicdispersers, Ultraturrax or high-performance mills (eg Attritor).

The process for applying a dispersion as a thin layer onto and into aflexible perforate support, such as a woven or nonwoven, to produce acomposite comprising this flexible perforate support and a porous firstceramic material which fills the perforations in the support and thepreparation of a composite comprising a perforate support and a porousceramic material whose pore structure is characterized by an averagepore size are known in principle from WO 99/15262. However, not all theparameters or ingredients, especially nonelectroconductive ingredients,can be used for producing the separator of the present invention. Inparticular, the ceramic particles which are used for producing thedispersion and fine particles and also the materials used as a flexibleperforate support differ from the ingredients described there.

The dispersion may be applied for example by printing on, pressing on,pressing in, rolling on, knifecoating on, spreadcoating on, dipping,spraying or pouring on onto and into the flexible perforate support. Thedispersion used for applying onto and into the flexible perforatesupport may comprise a sol of the elements Al, Zr and/or Si, and isproduced in this case by dispersing the ceramic particles and fineparticles in one of these sols. The sols are obtainable by hydrolyzingat least one compound with water or an acid or a combination of thesecompounds. It may be preferable to introduce the compound to behydrolyzed into an alcohol or an acid or a combination of these liquidsprior to hydrolysis. The compound to be hydrolyzed is preferably atleast one nitrate, chloride, carbonate, alkoxide or an organoelementcompound of the elements Al, Zr and/or Si. The hydrolysis is preferablycarried out in the presence of liquid water, water vapor, ice or an acidor a combination thereof.

In an embodiment of the process according to the invention, hydrolysisof the compounds to be hydrolyzed is used to prepare particulate sols.These particulate sols are notable for the compounds formed byhydrolysis being present in the sol in particulate form. Particulatesols can be prepared as described above or as in WO 99/15262. These solscustomarily have a very high water content, which is preferably above50% by weight. It may be preferable for the compound to be hydrolyzed tobe introduced into alcohol or an acid or combination of these liquidsprior to hydrolysis. The hydrolyzed compound may be peptized bytreatment with at least one organic or inorganic acid, preferably with a10-60% organic or inorganic acid, more preferably with a mineral acidselected from sulfuric acid, hydrochloric acid, perchloric acid,phosphoric acid and nitric acid or a mixture thereof. The particulatesols thus produced may subsequently be used for producing dispersions,in which case it is preferable to produce dispersions for application tofiber webs which have been pretreated with polymeric sol.

In a further embodiment of the process according to the invention,hydrolysis of the compounds to be hydrolyzed is used to preparepolymeric sols. In this preferred embodiment of the process according tothe invention, the sol has a water and/or acid fraction of less than 50%by weight. These polymeric sols are notable for the fact that thecompounds formed by hydrolysis are present in the sol in polymeric form,ie in the form of chains crosslinked across a relatively large space.Polymeric sols customarily include less than 50% by weight andpreferably much less than 20% by weight of water and/or aqueous acid. Toobtain the preferred fraction of water and/or aqueous acid, thehydrolysis is preferably carried out in such a way that the compound tobe hydrolyzed is hydrolyzed with from 0.5 to 10 times the molar ratioand preferably with half the molar ratio of liquid water, water vapor orice, based on the hydrolyzable group of the hydrolyzable compound. Theamount of water used can be up to 10 times in the case of compoundswhich are very slow to hydrolyze, such as tetraethoxysilane. Compoundswhich are very quick to hydrolyze, such as zirconium tetraethoxide, areperfectly capable under these conditions of forming particulate sols asit is, which is why it is preferable to use 0.5 times the amount ofwater to hydrolyze such compounds. A hydrolysis with less than thepreferred amount of liquid water, water vapor or ice likewise leads togood results, although using more than 50% less than the preferredamount of half the molar ratio is possible but not very sensible, sincehydrolysis would no longer be complete and coatings based on such solswould not be very stable using an amount below this value.

To produce sols having a desired very low fraction of water and/or acidin the sol, it may be preferable for the compound to be hydrolyzed to bedissolved in an organic solvent, especially ethanol, isopropanol,butanol, amyl alcohol, hexane, cyclohexane, ethyl acetate and/ormixtures thereof, before the actual hydrolysis is carried out. A solthus produced may be used for producing the suspension of the presentinvention.

Both the particulate sols (high water fraction, low solvent fraction)and polymeric sols (low water fraction, high solvent fraction) areuseful as a sol to produce the dispersion in the process of the presentinvention. Not just sols which are obtainable as just described can beused, but in principle also commercially available sols, for examplezirconium nitrate sol or silica sol. The process of producing separatorsby applying a suspension to and solidifying it on a support is known perse from DE 101 42 622 and in similar form from WO 99/15262, but not allthe parameters and ingredients are applicable to the production of themembrane of the present invention. More particularly, the operationdescribed in WO 99/15262 is in that form not fully applicable topolymeric nonwoven materials, since the very watery sol systemsdescribed therein frequently do not permit complete, in-depth wetting ofthe customarily hydro-phobic polymeric nonwovens, since most polymericnonwovens are only badly wetted by the very watery sol systems, if atall. It has been determined that even the minutest unwetted areas in thenonwoven material can lead to membranes or separators being obtainedthat have defects and hence are inutile.

It has now been found that, surprisingly, a sol system or dispersionwhose wetting behavior has been adapted to the polymers will completelypenetrate the nonwoven materials and so provide defect-free coatings. Inthe process of the present invention, it is therefore preferable toadapt the wetting behavior of the sol or dispersion. This is preferablyaccomplished by producing sols or dispersions comprising one or morealcohols, for example methanol, ethanol or propanol or mixtures thereof,and/or aliphatic hydrocarbons. But other solvent mixtures areconceivable as well for addition to the sol or suspension in order thatthe wetting behavior thereof may be adapted to the nonwoven used.

The mass fraction of the suspended component (metal oxide particles) inthe suspension is preferably from 1 to 100 times, more preferably from 1to 50 times and most preferably from 1 to 10 times that of the sol used.It is particularly preferable for the metal oxide particles used forpreparing the dispersion to be aluminum oxide particles which preferablyhave an average particle size from 0.1 to 10 μm, in particular from 0.5to 5 μm. Aluminum oxide particles in the range of the preferred particlesizes are available for example from Martinswerke under the designationsMDS 6; DN 206, MZS 3 and MZS 1 and from Alcoa with the designationCL3000 SG, CT800 SG and HVA SG.

It has been determined that the use of commercially available metaloxide particles may in certain circumstances lead to unsatisfactoryresults, since the particle size distribution is frequently very large.It is therefore preferable to use metal oxide particles which wereclassified by a conventional process, for example wind sifting,centrifugation and hydro-classification. It is preferable for the metaloxide particles used to be a fraction for which the coarse grainfraction, which accounts for up to 10% of the total amount, wasseparated off by wet sieving. This unwelcome coarse grain fraction,which is very difficult or impossible to comminute even by the typicalprocesses of slip production such as, for example, grinding (ball mill,attritor mill, pestle mill), dispersing (Ultra-Turrax, Ultrasound),trituration or chopping, can consist for example of aggregates, hardagglomerates, grinding media attritus. The aforementioned measuresensure that the inorganic porous layer has a very uniform pore sizedistribution. This is achieved in particular by using metal oxideparticles whose maximum particle size is preferably from ⅓ to ⅕ and morepreferably more than 1/10 of the thickness of the nonwoven used.

Table 1 hereinbelow gives an overview of how the choice of the variousaluminum oxides affects the porosity and the resulting pore size of therespective porous inorganic coating. To determine these data, thecorresponding slips (suspensions or dispersions) were prepared and driedand solidified as pure moldings at 200° C. TABLE 1 Typical data ofceramics as a function of powder type used Al₂O₃ type Porosity/% Averagepore size/nm AlCoA CL3000SG 51.0 755 AlCoA CT800SG 53.1 820 AlCoA HVA SG53.3 865 AlCoA CL4400FG 44.8 1015 Martinsw. DN 206 42.9 1025 Martinsw.MDS 6 40.8 605 Martinsw. MZS 1 + 47% 445 Martinsw. MZS 3 = 1:1 Martinsw.MZS 3 48% 690

To improve the adhesion of the inorganic components to polymeric fibersas a substrate, it can be advantageous for the suspensions used to beadmixed with adhesion promoters, for example organofunctional silanes.Useful adhesion promoters include in particular compounds selected fromthe octylsilanes, the vinylsilanes, the amine-functionalized silanesand/or the glycidyl-functionalized silanes, for example the Dynasilanesfrom Degussa. Particularly preferred adhesion promoters for polymericfibers such as polyethylene (PE) and polypropylene (PP) are vinyl-,methyl- and octylsilanes, although an exclusive use of methylsilanes isnot optimal, for polyamides and polyamines they are amine-functionalsilanes, for polyacrylates and polyesters they areglycidyl-functionalized silanes and for polyacrylonitrile it is alsopossible to use glycidyl-functionalized silanes. Other adhesionpromoters can be used as well, but they have to be adapted to therespective polymers. The adhesion promoters accordingly have to beselected so that the solidification temperature is below the melting orsoftening temperature of the polymer used as substrate and below itsdecomposition temperature. Dispersions according to the presentinvention preferably include distinctly less than 25% by weight and morepreferably less than 10% by weight of compounds capable of acting asadhesion promoters. An optimal fraction of adhesion promoter resultsfrom coating the fibers and/or particles with a mono-molecular layer ofthe adhesion promoter. The amount in grams of adhesion promoter requiredfor this purpose can be obtained by multiplying the amount in g of theoxides or fibers used by the specific surface area of the materials inm²g⁻¹ and then dividing by the specific area required by the adhesionpromoter in m²g⁻¹, the specific area required frequently being in theorder of from 300 to 400 m²g⁻¹.

Table 2 which follows contains an illustrative overview of usableadhesion promoters based on organofunctional silicon compounds fortypical nonwoven material polymers. TABLE 2 Polymer Organofunctionaltype Adhesion promoter PAN Glycidyl GLYMO Methacryloyl MEMO PA AminoAMEO, DAMO PET Methacryloyl MEMO Vinyl VTMO, VTEO, VTMOEO PE, PP AminoAMEO, AMMO Vinyl VTMO, VTEO, Silfin Methacryloyl MEMOwhere:

-   AMEO=3-aminopropyltriethoxysilane-   DAMO=2-aminoethyl-3-aminopropyltrimethoxysilane-   GLYMO=3-glycidyloxytrimethoxysilane-   MEMO=3-methacryloyloxypropyltrimethoxysilane-   Silfin=vinylsilane+initiator+catalyst-   VTEO=vinyltriethoxysilane-   VTMO=vinyltrimethoxysilane-   VTMOEO=vinyltris(2-methoxyethoxy)silane

In a particular embodiment of the process according to the presentinvention, the abovementioned adhesion promoters are applied to theflexible perforate support such as a polymeric nonwoven for example in apreceding step. To this end, the adhesion promoters are dissolved in asuitable solvent, for example ethanol. This solution may additionallyinclude a small amount of water, preferably from 0.5 to 10 times themolar amount of the hydrolyzable group, and small amounts of an acid,for example HCl or HNO₃, as a catalyst for the hydrolysis andcondensation of the Si—OR groups. This solution is applied to thesubstrate by the familiar techniques, for example spraying on, printingon, pressing on, pressing in, rolling on, knifecoating on, spreadcoatingon, dipping, spraying or pouring on, and the adhesion promoter is fixedon the substrate by a thermal treatment at from 50 to not more than 350°C. It is only after the adhesion promoter has been applied in thisembodiment of the process according to the present invention that thedispersion is applied and solidified.

Application of an adhesion promoter prior to the actual application ofthe dispersion provides improved adhesivity of the flexible substratesespecially with regard to aqueous particulate sols, which is whyespecially thus pretreated substrates can be coated according to thepresent invention with suspensions based on commercially available sols,for example zirconium nitrate sol or silica sol. But this way ofapplying an adhesion promoter also means that the production process ofthe separator according to the present invention has to be extended toinclude an intervening or preliminary treatment step. This is feasiblealbeit more costly and inconvenient than the use of adapted sols towhich adhesion promoters have been added, but also has the advantagethat better results are obtained even on using dispersions based oncommercially available sols.

The coatings according to the invention are applied into and onto thesubstrate by solidifying the dispersion in and on the substrate and ontoat least one side of a layer of porous ceramic material. According tothe present invention, the dispersion present on and in the substratecan be solidified by heating at from 50 to 350° C. Since the maximumtemperature is dictated by the polymeric nonwoven used when polymericsubstrate materials are used, the maximum temperature must be adaptedaccordingly. Thus, depending upon the embodiment of the processaccording to the present invention, the dispersion present on and in thenonwoven is solidified by heating at from 100 to 350° C. and mostpreferably by heating at from 110 to 280° C. It can be advantageous forthe heating to take place at from 100 to 350° C. for from 1 second to 60minutes. It is more preferable to solidify the dispersion by heating atfrom 110 to 300° C. and most preferably at from 110 to 280° C. andpreferably for from 0.5 to 10 min.

The assembly may be heated according to the present invention by meansof heated air, hot air, infrared radiation or by other heating methodsaccording to the prior art.

The process according to the present invention can be carried out forexample by unrolling the flexible substrate for example a polymericnonwoven off a roll, passing it at a speed of from 1 m/h to 2 m/s,preferably at a speed of from 0.5 m/min to 20 m/min and most preferablyat a speed of from 1 m/min to 5 m/min through at least one apparatuswhich applies the suspension atop and into the substrate, for example aroller, and at least one further apparatus whereby the dispersion issolidified on and in the support by heating, for example an electricallyheated furnace, and rolling the separator thus produced up on a secondroll. This makes it possible to produce the separator according to thepresent invention in a continuous process. Similarly, the pretreatmentsteps can be carried out on a continuous basis by observing theparameters mentioned.

It has been determined to be particularly advantageous for the processto be carried out with the substrate and especially the polymericnonwoven having a maximum tension of 10 N/cm and preferably of 3 N/cm inthe longitudinal direction during the coating operation or operations.The term “coating operations” refers in this context to all processsteps in which a material is brought onto and into the substrate and issolidified there by heat treatment, ie including the application of theadhesion promoter. Preferably, the substrate is tensioned with a maximumforce of 0.01 N/cm during the coating operations. It may be particularlypreferable for the substrate to be tensionless in the longitudinaldirection during the coating operation or operations.

The pulling tension can be controlled during the coating in order thatno deformation, even an elastic one, of the carrier material may takeplace. Possible deformation (stretching) due to excessive pullingtension can mean that the ceramic coating cannot follow the substratematerial of construction, the consequence being that the coating willbecome detached from the nonwoven material over the entire area. Theresulting product can then not be used for the intended purpose.

The separator according to the present invention may be equipped with anadditional automatic shutdown mechanism by, for example, applying alayer of particles which, at a desired temperature, melt and close thepores of the separator, so-called shutdown particles, to the separatorafter the solidification of the applied dispersion on the substrate tocreate a shutdown mechanism, and fixing the layer of shutdown particles.The layer of shutdown particles can be created for example by applying asuspension of waxy particles having an average particle size larger thanthe average pore size of the separator in a sol, water, solvent orsolvent mixture.

The suspension for applying the particles contains preferably from 1 to50% by weight, more preferably from 5 to 40% by weight and mostpreferably from 10 to 30% by weight of shutdown particles, especiallywax particles, in the suspension.

Since the inorganic coating on the separator frequently has a veryhydrophilic character, it has been determined to be advantageous for thecoating on the separator to be prepared using a silane in a polymericsol as an adhesion promoter and thus be hydro-phobicized. To achievegood adhesion and uniform dissipation of the shutdown particles in theshutdown layer on hydrophilic as well as hydrophobic porous inorganicseparator layers, there are several possibilities.

In one version of the process according to the present invention, it hasbeen determined to be advantageous to hydrophobicize the porousinorganic layer of the separator before the shutdown particles areapplied. The production of hydrophobic membranes which works accordingto the same principle is described in WO 99/62624 for example.Preferably, the porous inorganic coating is hydrophobicized by treatmentwith alkyl-, aryl- or fluoroalkylsilanes marketed for example by Degussaunder the tradename of Dynasilane. It is possible in this context toemploy for example the familiar hydrophobicization methods which areemployed inter alia for textiles (D. Knittel; E. Schollmeyer; MelliandTextilber. (1998) 79(5), 362-363), with minimal changes to the recipes,for the porous coatings on the separator as well. To this end, thecoating or separator is treated with a solution which includes at leastone hydrophobic material. It can be advantageous for the solvent in thesolution to be water, preferably adjusted to a pH of 1-3 with an acid,preferably acetic acid or hydrochloric acid, and/or an alcohol,preferably ethanol. The solvent fraction attributable to acid-treatedwater or to alcohol can be in each case from 0% to 100% by volume.Preferably the fraction of the solvent which is attributable to water isin the range from 0% to 60% by volume and the fraction of solvent whichis attributable to alcohol in the range from 40% to 100% by volume. Thesolvent has introduced into it from 0.1% to 30% by weight and preferablyfrom 1% to 10% by weight of a hydrophobic material to prepare thesolution. Useful hydrophobic materials include for example theabove-recited silanes. Surprisingly, good hydrophobicization is obtainednot just with strongly hydrophobic compounds such as for exampletriethoxy-(3,3,4,4,5,5,6,6,7,7,8,8-tridecafluorooctyl)silane, but atreatment with methyltriethoxysilane or i-butyl-triethoxysilane iscompletely sufficient to obtain the desired effect. The solutions arestirred at room temperature to achieve uniform dissipation of thehydrophobic materials in the solution and subsequently applied to theinorganic coating on the separator and dried. Drying can be speeded upby treatment at temperatures from 25 to 100° C.

In a further version of the process according to the present invention,the porous inorganic coating can also be treated with other adhesionpromoters before the shutdown particles are applied. The treatment withone of the hereinbelow mentioned adhesion promoters can then likewise beeffected as described above, ie by treating the porous inorganic layerwith a polymeric sol which includes a silane adhesion promoter.

The layer of shutdown particles is preferably created by applying to theinorganic coating on the separator a suspension of shutdown particles ina suspension medium selected from the group consisting of a sol, water,solvents, for example alcohol, ether or ketones, and a solvent mixtureand then drying. The particle size of the shutdown particles present inthe suspension is arbitrary in principle. However, it is advantageousfor the suspension to include shutdown particles having an averageparticle size (D_(w)) of not less than and preferably greater than theaverage size of the pores of the porous inorganic layer (ds), since thisensures that the pores of the inorganic layer are not clogged byshutdown particles in the course of the production of the separatoraccording to the present invention. The shutdown particles usedpreferably have an average particle size (D_(w)) which is greater thanthe average pore diameter (d_(s)) and less than 5 d_(s) and morepreferably less than 2 d_(s).

To employ shutdown particles smaller in size than the pores of theporous inorganic layer, the particles must be prevented from penetratinginto the pores of the porous inorganic separator layer. Reasons foremploying such particles include for example large price differences,but also availability. One way of preventing the penetration of shutdownparticles into the pores of the porous inorganic layer is to control theviscosity of the suspension in such a way that absent external shearingforces no penetration of the suspension into the pores of the inorganiclayer on the separator takes place. Such a high viscosity for thesuspension is obtainable for example by adding auxiliaries whichinfluence the flow behavior, for example silicas (Aerosil, Degussa), tothe suspension. When auxiliaries are used, for example Aerosil 200, afraction from 0.1% to 50% by weight and preferably from 0.5% to 10% byweight of silica, based on the suspension, will frequently be sufficientto achieve a sufficiently high viscosity for the suspension. Thefraction of auxiliaries can in each case be determined by simplepreliminary tests.

It can be advantageous for the suspension used, which contains shutdownparticles, to contain adhesion promoters. Such a suspension withadhesion promoter can be applied directly to an inorganic layer of theseparator even when the layer was not hydrophobicized beforehand. Itwill be appreciated that a suspension with adhesion promoter can also beapplied to a hydrophobicized layer or to a separator layer which hasbeen made employing an adhesion promoter. Adhesion promoters useful inthe shutdown particle suspension are preferably silanes having amino,vinyl or methacryloyl side groups. Such adhesion promoters include forexample AMEO (3-aminopropyltriethoxy-silane), MEMO(3-methacryloyloxypropyltrimethoxy-silane), Silfin(vinylsilane+initiator+catalyst), VTEO (vinyltriethoxysilane) or VTMO(vinyltrimethoxy-silane). Such silanes are available for example fromDegussa as an aqueous solution under the designation Dynasilane 2926,2907 or 2781. An adhesion promoter fraction of not more than 10% byweight has been determined to be sufficient for ensuring sufficientadhesion of the shutdown particles to the porous inorganic layer.Shutdown particle suspensions with adhesion promoter preferably containfrom 0.1% to 10% by weight, more preferably from 1% to 7.5% by weightand most preferably from 2.5% to 5% by weight of adhesion promoter,based on the suspension.

Useful shutdown particles include all particles having a defined meltingpoint. The particle material is chosen according to the desired shutdowntemperature. Since relatively low shutdown temperatures are desired formost batteries, it is advantageous to use shutdown particles selectedfrom particles of polymers, polymer blends, natural and/or artificialwaxes. Particularly preferred shutdown particles are particles ofpoly-propylene or polyethylene wax.

The shutdown particle suspension may be applied to the porous inorganiclayer of the separator by printing on, pressing on, pressing in, rollingon, knifecoating on, spreadcoating on, dipping, spraying or pouring on.The shutdown layer is preferably obtained by drying the appliedsuspension at a temperature from room temperature to 100° C. andpreferably from 40 to 60° C.

It may be preferable for the shutdown particles to be fixed after theyhave been applied to the porous inorganic layer, by heating one or moretimes to a temperature above the glass transition temperature, so thatthe particles are fused on without undergoing a change in the actualshape. This makes it possible to ensure that the shutdown particlesadhere particularly firmly to the porous inorganic separator layer.

The applying of the shutdown particle suspension with subsequent dryingand any heating to above the glass transition temperature can be carriedout continuously or quasicontinuously. When the starting material usedis a flexible separator it can again be unwound off a roll, passedthrough a coating, drying and, if used, heating apparatus and then berolled up again.

The invention also provides an electrochemical cell, especially alithium battery, lithium ion battery or a lithium polymer battery,comprising one of the above-described separators.

The electrolyte which is used in such an electrochemical cell can be anycustomary electrolyte which can be used in electrochemical cells.Examples which can be mentioned include solutions of a soluble lithiumsalt in one or more organic solvents, for example ethylene carbonate anddimethyl carbonate (EC-DMC). Other suitable nonaqueous solvents includefor example γ-butyrolactone, tetrahydrofuran, 1,2-dimethoxyethane,propylene carbonate, diethyl carbonate, methyl ethyl carbonate,diethoxyethane, dioxolane and methyl formate. Suitable soluble lithiumsalts are those customarily used. Examples which may be mentionedinclude LiPF₆, LiAsF₆, LiBF₄, LiClO₄, LiCF₃SO₃, LiN(CF₃SO₂)₃ andLiN(C₂F₅SO₂)₃, of which LiPF₆ is particularly preferred.

The present invention also includes the use of an inventive separatorfor producing an electrochemical cell, especially a lithium battery,lithium ion battery or a lithium polymer battery, each preferably forhigh current applications.

Preferably, the electrochemical cell is rechargeable.

By average pore size and porosity is meant the average pore size and theporosity which can be determined by the familiar method of mercuryporosimetry using a 4000 porosimeter from Carlo Erba Instruments.Mercury porosimetry rests on the Washburn equation (E. W. Washburn,“Note on a Method of Determining the Distribution of Pore Sizes in aPorous Material”, Proc. Natl. Acad. Sci., 7, 115-16 (1921)).

INVENTIVE, TEST AND REFERENCE EXAMPLES Reference Example 1 S450PETSeparator

To 130 g of water and 15 g of ethanol are initially added 30 g of a 5%by weight aqueous HNO₃ solution, 10 g of tetraethoxysilane, 2.5 g ofmethyltriethoxysilane and 7.5 g of the Dynasilane GLYMO. This sol, whichwas initially further stirred for some hours, was then used to suspend125 g each of the Martoxid MZS-1 and Martoxid MZS-3 aluminum oxides, inorder to obtain a slip (suspension). This slip is homogenized with amagnetic stirrer for at least a further 24 h, during which the stirredvessel has to be covered over in order that no solvent may be lost.

The above slip is then used to coat a 56 cm wide PET nonwoven having athickness of about 13 μm and a basis weight of about 6 g/m² using acontinuous roller coating process at a belt speed of about 30 m/h andT=200° C. This results in a separator having an average pore size of 450nm, which possessed very good adhesion and a thickness of about 30 μm.

Test Example 1 Lithium Battery with S450PET Separator from ReferenceExample 1

The S450PET separator produced in reference example 1 is installed in alithium ion cell consisting of a positive mass of LiCoO₂, a negativemass consisting of graphite and an electrolyte of LiPF₆ in ethylenecarbonate/dimethyl carbonate (EC/DMC) [LiCoO₂//S450PET, EC/DMC 1:1, 1MLiPF₆//graphite]. The charging behavior of this battery was tested.After more than 250 cycles, the battery exhibits a clear drop incapacity of up to 25%.

The capacity of this battery decreased even more distinctly after 200cycles when the cell is stored at elevated temperature (50-60° C.) inthe fully charged, partially charged or discharged state between thecycles.

Production of Inventive Separators

Inventive Example 1 Production of an Inventive S100PET Separator

To 145 g of water are initially added 30 g of a 5% by weight aqueous HClsolution, 10 g of tetraethoxysilane, 2.5 g of methyltriethoxysilane and7.5 g of the Dynasilane GLYMO. The sol obtained in this manner, whichwas initially further stirred for some hours, is then used to suspend140 g of the AlCoA CT3000 aluminum oxide and 7 g of Aerosil Ox50. Thissuspension is homogenized with a magnetic stirrer for at least a further72 h, during which the stirred vessel has to be covered over in orderthat no solvent may be lost.

The above suspension is then used to coat a 56 cm wide PET nonwovenhaving a thickness of about 13 μm and a basis weight of about 6 g/m²using a continuous roller coating process at a belt speed of about 30m/h and T=200° C. This provides a separator having an average pore sizeof 80 nm, which possesses very good adhesion and a thickness of about 24μm.

Inventive Example 2 Production of an Inventive S240 PF-T Separator

To 140 g of water and 10 g of ethanol were initially added 30 g of a 5%by weight aqueous HCl solution, 10 g of tetraethoxysilane, 2.5 g ofmethyltriethoxysilane and 7.5 g of the Dynasilane GLYMO. The solobtained in this manner, which was initially further stirred for somehours, was then used to suspend 265 g of the AlCoA CT1200 aluminum oxideand 3.3 g Li₁₃Al_(0.7)Ti_(1.4)(PO₄)₃. This suspension (slip) ishomogenized with a magnetic stirrer for at least a further 24 h, duringwhich the stirred vessel has to be covered over in order that no solventmay be lost.

The above suspension is then used to coat a 56 cm wide PET nonwovenhaving a thickness of about 13 μm and a basis weight of about 6 g/m² ina continuous roller coating process at a belt speed of about 30 m/h andT: 200° C. This provides a separator having an average pore size of 240nm, which possesses very good adhesion and a thickness of about 27 μm.

Inventive Example 3 Production of an Inventive S450PET Separator

A separator produced as per reference example 1 is coated with anaqueous 1% by weight suspension of finely divided ZrO₂ (VPH, Degussa AG)in a continuous impregnating operation and dried at 210° C.

Inventive Example 4 Production of an Inventive S800PET Separator

A separator produced as per reference example 1 is coated with ananhydrous ethanolic 1% by weight suspension of finely divided Li₃N in acontinuous impregnating operation and dried at 210° C.

Test Example 2 Lithium Battery with S450PET Separator

The S450PET separator produced in example 3 was installed in a lithiumion cell consisting of a positive mass of LiCoO₂, a negative massconsisting of graphite and an electrolyte of LiPF₆ in ethylenecarbonate/dimethyl carbonate (EC/DMC) [LiCoO₂//S450PET, EC/DMC 1:1, 1MLiPF₆//graphite]. The charging behavior of this battery was tested.After more than 500 cycles, the battery exhibited only a very minimaldrop in capacity of a few percentage points. Even increasing thecharging voltage from 4.1 to 4.2 in the 450th charging cycle did notharm the battery.

The capacity of the cells decreases only very moderately even on storageof this battery at elevated temperature (50-60° C.) in the fullycharged, partially charged or discharged state between the cycles.Batteries which are equipped with such a separator are thus very usefuleven in applications where heating to above 50° C. cannot be ruled out.

1. A separator for an electrochemical cell, comprising: (A) a flexibleperforate support, (B) a porous first ceramic material which fills theperforations in the support and which (i) has a pore structure having anaverage pore size, and (ii) is suitable for receiving an ion-conductingelectrolyte, wherein (C) an electrolyte-contactable pore surface of thefirst porous ceramic material is covered with fine particles of afurther material to extend the use life, the average size of the fineparticles being in the range from 0.5 to 30% of the average pore size ofthe ceramic material.
 2. The separator of claim 1, wherein the materialof the fine particles is identical to or different from the porousceramic material.
 3. The separator of claim 2, wherein the material ofthe fine particles is different from the porous ceramic material.
 4. Theseparator of claim 2, wherein the fine particles comprise SiO₂, Al₂O₃,ZrO₂ or SiC.
 5. The separator of claim 2, wherein the fine particlescomprise Li₂CO₃, Li₃N, LiAlO₂ or Li_(x)Al_(y)Ti_(z)(PO₄)₃, and wherein1≦x≦2, 0≦y≦1 and 1≦z≦2.
 6. The separator of claim 1, comprising anelectrolyte for ion conductance.
 7. The separator of claim 1, whereinthe fine particles are incorporated in the porous first ceramic materialand are exposed on the pore surface.
 8. The separator of claim 1,wherein the porous first ceramic material is coated with the fineparticles.
 9. The separator of claim 1, wherein the ceramic material hasan average pore size in the range from 50 nm to 5 μm.
 10. The separatorof claim 1, wherein the porous ceramic material comprising fineparticles has a porosity in the range from 10% to 70%.
 11. The separatorof claim 1, wherein the ceramic material comprises an oxide ofzirconium, silicon or preferably aluminum.
 12. The separator of claim 1,wherein the first ceramic material is produced by solidifying a slipwhich contains particles having a large average particle size whichdetermine the pore structure of the ceramic material and also particleshaving a smaller average primary particle size which adhere the largeparticles together in the course of the solidification of the slip. 13.The separator of claim 1, wherein the perforate support comprisespolymeric fibers, glass or ceramic.
 14. The separator of claim 1,wherein the perforate support comprises fibers.
 15. The separator ofclaim 1, wherein the support comprises fibers and/or filaments from 1 to150 μm and/or yarn from 3 to 150 μm in diameter.
 16. The separator ofclaim 1, wherein the support is a nonwoven having a pore size from 5 to500 μm.
 17. The separator of claim 1, wherein the separator is stableunder service conditions at not less than 100° C.
 18. The separator ofclaim 1 wherein the separator ranges, from 10 to 1 000 μm in thickness.19. The separator of claim 1, wherein the separator tolerates a bendingradius down to 100 mm.
 20. A process for producing a separator for anelectrochemical cell as claimed in claim 1, comprising: (a) applying adispersion as a thin layer onto and into a woven and/or nonwoven, thedispersion comprising (a1) large ceramic particles whose averageparticle size provides a pore structure to the thin layer having anaverage pore diameter, (a2) fine particles whose average particle sizeis in the range from 0.5 to 30%, of the average particle size of theceramic material, and (a3) optionally, ceramic particles having anaverage primary particle size which is substantially less than theaverage particle size of the ceramic particles as per (a1) and (a2); and(b) solidifying the dispersion at a temperature from 100° C. to 680° C.to form a separator.
 21. The process of claim 20, wherein the dispersionin step (a) further comprises a sol.
 22. A process for producing aseparator for an electrochemical cell as claimed in claim 1, comprising:(i) providing a composite formed from a perforated support, and also aporous ceramic material whose pore structure having an average poresize; (ii) treating the composite with a dispersion of fine particleshaving an average particle size in the range from 0.5 to 30% of theaverage pore size in a dispersion medium so that theelectrolyte-accessible pore surface of the composite is coated with thedispersion and the dispersion comprises from 1 to 25% by weight; and(iii) drying the dispersion at a temperature in the range from 100° C.to 680° C. so that the coated pore surface is coated with the fineparticles.
 23. The process of claim 22, wherein the composite is aseparator which is obtained by the process of claim
 20. 24. The processof claim 20, wherein the dispersion comprises one or more additionalcomponents selected from the group consisting of adhesion promoters,dispersing assistants, agents for setting the viscosity, agents forsetting the flow properties and other customary assistants for producingdispersions.
 25. The process of claim 20, wherein the dispersion mediumcontains water and the fine particles are hydrolysis-stable elementoxide particles.
 26. The process of claim 20, wherein the dispersionmedium is an anhydrous organic solvent and the fine particles comprisehydrolysis-sensitive materials.
 27. The process of claim 20, wherein theceramic particles comprise a material selected from the group consistingof aluminum oxide, silicon oxide, zirconium oxide and mixtures thereof.28. An electrochemical cell, a lithium battery, lithium ion battery or alithium polymer battery, wherein the cell comprises a separator asclaimed in claim
 1. 29. (canceled)